1. Field of the Invention
This invention relates to electric signal amplifiers.
2. Description of Related Art
An amplifier required to impress an amplified version of an electric input signal on a load must have an output stage with adequate current and voltage capability. For loads requiring a high voltage/current product a simple amplifier has to dissipate high power in its output stage. In many applications this dissipation is intolerable and means to reduce the dissipation are sought. Beyond reducing the quiescent power in the output stage to near zero by so-called Class B operation, there are techniques that switch higher supply voltages to the output stage only when the output voltage is required to be high. Such techniques reduce dissipation by dropping as little voltage as possible across the output stage active devices. While satisfactory for medium and high voltages these techniques offer little benefit for low voltage, high current applications where load voltages are comparable with voltages dropped across the active devices. Furthermore, operating linear power active active devices at low voltages can often impair their gain and dynamic properties.
One known form of electric amplifier which can satisfactorily be used for low voltage, high current applications comprises: an output stage arranged to control the voltage supplied to the load in dependence on an input signal applied to the amplifier; means for separately supplying to said load a current which changes at a rate faster than the maximum rate of change of current in the load required by the input signal, thereby to produce a corresponding change in the current required to be supplied by the output stage to the load in order to maintain the voltage across the load at the value required by the input signal; and switching means operated in dependence on the value of the current supplied to the load by said output stage so as to control the sense in which the changing current changes, thereby to restrict the current supplied to the load by the output stage to values less than the total current required by the input signal.
Such an amplifier is shown in UK Patent Application No. GB 2070373-A. This amplifier uses a switching means comprising two switching circuits, one of which serves to restrict the current supplied by the output stage to values between two predetermined positive levels for positive input signals and the other of which restricts the current supplied by the output stage to values between two predetermined negative values for negative input signals.